A critical step in the art of making fiberglass is fiberizing. In the fiberizing step, molten glass flows from a forehearth to a bushing assembly where it flows through a plurality of orifices. Generally, 200 to 4,000 or more orifices are formed in the bushing plate depending upon the type of product, desired throughput and manufacturing equipment constraints. The orifices are defined by elongated nozzles attached to or formed in the bushing assembly.
A vital function of a bushing assembly is its ability to maintain the glass melt in the bushing above the fiberizing tips at controlled temperature, nominally up to 2150.degree. F. or higher, depending on glass composition appropriate for fiberizing glass. Prior art bushings include rows of fiberizing nozzles arrayed in a series of columns. The nozzles are cooled by circulating water in a series of tubes located near the nozzles or by cooling fins. Ideally, each of the nozzles is kept at the same temperature with no hot spots or cold spots. Hot spots on the bushing can cause uneven flow and fiber breakage. Cold spots on the bushing result in freeze-up and a reduction in bushing throughput efficiency.
In prior art bushings currently in use, electrical current flows through platinum bushing material which functions to properly distribute heat. Prior art bushings must be carefully constructed with the thickness of the bushing walls varying substantially so that current flow can be controlled. Use of a bushing having a standard wall thickness was precluded by the need to provide even heat. Bushings are formed by welding platinum plates of varying thicknesses together depending upon the electrical resistance characteristics of the bushing design and desired thermal characteristics.
Platinum alloy bushings are used because of the temperature resistance characteristics and durability of platinum. Due to the high cost of platinum alloys, it is desireable to minimize the use of platinum alloy in the bushing assembly.
The nozzles are elongated and normally are about 1/16 to 1/4 of an inch in length. The elongated nozzles allow the molten glass to cool as it passes through the nozzle. The nozzle extends below the lower surface of the bushing plate to also reduce the tendency of the molten glass to flood along the bottom side of the bushing plate.
The nozzles are generally arranged in a series of rows which span the entire lower surface of the bushing plate. One problem associated with the arrangement of the nozzles in a series of closely spaced rows is that when a fiber break occurs, a bead formed on one nozzle can interfere with the formation of adjacent fibers causing them to break in a chain reaction.
Heat loss through the exposed surface of the bushing is substantial and adds to energy usage costs.
Bushings must be replaced periodically due to wear and heat sag. Heat sag is a term used in the art to refer to the tendency of bushings to sag over time becoming distorted in shape. The nozzles may be welded to holes formed in a fiberizing plate or formed from the plate by extruding or forming operations.
Electrical induction heated bushings are not known to be currently in general use in the industry. Induction bushings previously used were generally round and operated by applying inductive current through the bushing from the outside. Problems encountered with this approach included difficulty in heating control, uneven heating and interference with heating from metal masses. Induction heated bushings required thick platinum alloy walls, and required application of substantial electrical energy. Capacity of these bushings was fairly limited.
A primary object of the present invention is to provide a bushing which has lower platinum alloy requirements per unit of production.
Another primary object of the present invention is to provide a bushing having lower energy usage requirements.
Another object of the present invention is to provide a bushing which uses radiant heat to provide even heat distribution and offers more consistent glass fiber diameter and quality. Ceramic bodies are preferably used to retain the radiant heat from heating elements contained in the ceramic bodies which also insulate the bushing.
An object of the present invention is to provide a bushing which is not adversely affected by heat sag after an extended period of use.
One object of the present invention is to provide a bushing which is easier to manufacture and minimizes the need for welding operations when forming a bushing. The bushing may be formed from a panel having substantially uniform thickness due to the elimination of heating by passing electrical current through the bushing.
A more specific object of the invention is a method of making a fiberizing bushing assembly for attachment to an opening of a forehearth of a glass furnace to manufacture fiberglass. The method comprises the steps of forming a bushing by hydroforming a sheet metal blank to form a depression having an extrusion face; forming a plurality of holes in the extrusion face; forming a tip on each of the holes which extends from one side of the extrusion face; machining the ends of the tips to a uniform length; attaching ceramic blocks having resistance heating elements embedded therein to the lower surface and vertical perimeter portions of the bushing; attaching the bushing and ceramic blocks to the opening of the forehearth with a frame; and, mounting cooling means on the frame adjacent the tips for cooling glass fibers upon exiting the tips.
Another object of the invention is to provide a bushing made by less expensive forming processes to minimize the need for welding and expensive machining operations.
Another more specific object of the invention is to provide a fiberizing bushing assembly for making fiberglass wherein molten glass is heated in a forehearth and supplied through an opening to the fiberizing bushing assembly. The bushing assembly comprises a frame, a bushing having a perimeter flange engaged by the frame to retain the bushing on the opening in the forehearth. An outside wall of the bushing extends downwardly from the flange in a continuous loop having an upper edge contiguous with the flange and a lower edge spaced from the flange. An extrusion face extends substantially parallel to the perimeter flange and inwardly from the outside wall at the lower edge of the outside wall to an inner edge. A base wall extends between and interconnects the area between the inner edges of the extrusion face. One or more heater blocks are secured to the bushing for heating the outside wall and the base wall to maintain the temperature of molten glass within the bushing at the desired fiberizing temperature. A plurality of tips are formed in the extrusion face of the bushing through which molten glass is drawn to form continuous glass fibers or filaments. The tips each define a hole through the extrusion face and extend slightly below the extrusion face. Preferably, cooling tubes or fins are located adjacent the extrusion face for cooling the glass filaments upon existing the tips.
These and other problems are overcome and objects are obtained by the glass fiberizing bushing of the present invention and the method of making such a bushing as will be summarized below.